1. Field of the Invention
The present invention generally relates to the art of optoelectronic sensing, and more specifically to an improved fiber optic sensor for sensing the pressure in a high temperature, high pressure environment such as the combustion chamber of an internal combustion engine.
2. Description of the Related Art
U.S. Pat. No. 4,799,751, entitled "DETECTION DEVICE USING FIBER OPTIC TECHNIQUES" issued Jan. 24, 1989 to V Tekippe, discloses a pressure sensor such as illustrated in FIGS. 1a to 1c and 2 and designated as 10. The sensor 10 includes a flexible disc or diaphragm 12 having an upper first surface 12a which is exposed to a pressure to be sensed.
Light is injected into the lower end of a first or center optical fiber 14 as indicated by an intensity I.sub.0, and propagates upwardly through and is projected by the fiber 14 onto the second surface 12b of the diaphragm 12. The light path through fiber 14 to the diaphragm is indicated by hatching. Second and third outer optical fibers 16 and 18 are spaced on the opposite sides of the fiber 14. The fibers 14, 16 and 18 are parallel, at least at their upper end portions which are adjacent to the diaphragm 12.
Light which emerges from the upper end of the fiber 14 is reflected from the surface 12b and enters the upper ends of the fibers 16 and 18 as indicated by downward directed arrows. The reflected light propagating through the fibers 16 and 18 is designated as intensities I.sub.IN and I.sub.OUT respectively.
The center of the inner fiber 14 is spaced from the center of the diaphragm 12 by a distance x. The fiber 16 is radially inward of the fibers 14 and 18. Preferably, x=r/3.sup.1/2 where r is the radius of the diaphragm 12. At this point the radial slope of the diaphragm 12 exhibits a maximum and provides the sensor 10 with maximum sensitivity. The upper ends of the fibers 14, 16 and 18 are spaced from the surface 12b of the diaphragm 12 by a distance y and are perpendicular to the diaphragm 12 when the diaphragm 12 is flat.
FIG. 1b illustrates the case in which the pressure P which is applied to the surface 12a of the diaphragm 12 is zero, and the diaphragm 12 is flat. The upper ends of the fibers 14, 16 and 18 are all spaced from the diaphragm 12 by the distance y, and the same amount of light is reflected from the surface 12b of the diaphragm 12 into both fibers 16 and 18 such that I.sub.IN =I.sub.OUT.
FIG. 1a illustrates the pressure P as being greater than zero. The diaphragm 12 is curved by the applied pressure such that the surface 12b which faces the fibers 14, 16 and 18 is convex. Since the fibers 14, 16 and 18 are offset from the center of the diaphragm 12, more light is reflected from the surface 12b into the fiber 18 than into the fiber 16. In this case I.sub.IN &lt;I.sub.OUT.
When the pressure P is less than zero as illustrated in FIG. 1c, the surface. 12b of the diaphragm 12 which faces the fibers 14, 16 and 18 is concave, and more light is reflected from the surface 12b into the fiber 16 than into the fiber 18, such that I.sub.IN &gt;I.sub.OUT. In this manner, the sensor 10 is capable of sensing both the magnitude and sign (positive or negative) of the pressure P.
The geometry of the sensor 10 for the exemplary case of negative pressure P is illustrated in FIG. 2. As disclosed by Tekippe, the fibers 14, 16 and 18 are identical, having the same diameter, index of refraction and numerical aperture. Assuming that the index of refraction of the cores of the fibers 14, 16 and 18 is n.sub.1 and the index of refraction of the cladding of the fibers 14, 16 and 18 is n.sub.2, the critical angle .theta..sub.c, below which total internal reflection will occur in the fibers 14, 16 and 18, is .theta..sub.c =sin.sup.-1 (n.sub.2 /n.sub.1).
The slope of the diaphragm 12 at the center of the light beam incident on the surface 12b from the center fiber 14 is .DELTA.y/.DELTA.X, and the curvature of the diaphragm 12 can be expressed as an angle of inclination .phi.=tan.sup.-1 -(.DELTA.y/.DELTA.X). Light emerging from the fiber 14 at the critical angle .theta..sub.c is refracted in the air gap between the fibers 14, 16 and 18 and the diaphragm 12 and is incident on the surface 12b of the diaphragm 12 at an angle .theta..
Due to the curvature of the diaphragm 12, this light is reflected from the surface 12b of the diaphragm 12 into the upper end of the fiber 16 at an angle .theta..sub.1 =.theta.+.phi., and light is reflected into the upper end of the fiber 18 at an angle .theta..sub.1 =.theta.-.phi..
Assuming that the diameter of the diaphragm 12 is much larger than the diameter of the fiber 14, the pressure P can be expressed as the ratio of the sum and difference of the intensities I.sub.IN and I.sub.OUT as (I.sub.OUT -I.sub.IN)/(I.sub.OUT +I.sub.IN)=2AP. Solving for P produces P=(1/2A)[(I.sub.OUT -I.sub.IN)/(I.sub.OUT +I.sub.IN)]. For low temperature applications, A is substantially constant.
The sensed pressure P is therefore independent of the diameter of the fibers 14, 16 and 18, the distance y between the fibers 14, 16 and 18 and the diaphragm 12, the reflectance of the diaphragm 12, the intensity of the input light I.sub.0, and is therefore insensitive to environmental perturbations.
The prior art arrangement of Tekippe is subject to substantial optical signal noise caused by physical perturbation or bending of the fibers 14, 16 and 18. Bending of the fibers 14, 16 and 18 causes changes in the optical mode distribution of the light propagating therethrough, which in turn affects the intensities I.sub.IN and I.sub.OUT and thereby the value of P.
Accurate sensing of the pressure in the combustion chamber of an internal combustion engine enables advantageous feedback control of spark advance, cylinder dilution, individual cylinder fuel rate and other parameters using pressure ratio management algorithms. At high temperatures on the order of 700.degree. C., such as encountered in this type of application, the value of A varies significantly with temperature.
More specifically, A is a function of the Young's modulus and Poisson's ratio of the diaphragm 12, both of which are temperature dependent. This causes the sensitivity of the diaphragm 12 to vary with temperature, and produce inaccuracy in the sensed value of pressure P.
A diaphragm 12 consisting of a simple disc as disclosed by Tekippe is subject to temperature induced curvature which also varies with temperature. The curvature of such a diaphragm 12 will change as the temperature increases or decreases, even if there is no change in pressure P. These effects cause zero drift in the pressure reading, and limit the attainable accuracy of the sensor 10 in a high temperature environment.